Production of Inorganic Doped-Zircon Pigments

ABSTRACT

A process for producing an inorganic doped-zircon pigment includes calcining a base mixture comprising raw plasma-dissociated zircon, a chromophore, and at least one mineralizer, to produce a raw pigment. The raw pigment is refined to obtain an inorganic doped-zircon pigment.

THIS INVENTION relates to the production of inorganic doped-zirconpigments. It relates in particular to a process for producing aninorganic doped-zircon pigment.

According to the invention, there is provided a process for producing aninorganic doped-zircon pigment, which process includes

-   -   calcining a base mixture comprising raw plasma-dissociated        zircon, a chromophore, and at least one mineralizer, to produce        a raw pigment; and    -   refining the raw pigment to obtain an inorganic doped-zircon        pigment.

By ‘raw plasma dissociated zircon or PDZ’ is meant PDZ that has beenobtained directly by means of plasma dissociation, ie without anytreatment thereof between the plasma dissociation of the zircon and themixing of the resultant PDZ with the chromophore and the mineralizer,being effected. In particular, the process is characterized thereby thatthe raw PDZ is not subjected to any milling and/or any chemicaltreatment prior to its use in forming the base mixture.

The raw PDZ may be that obtained by generating a high temperature plasmazone, and feeding particulate zircon, ZrSiO₄, into the plasma zone,thereby to dissociate the zircon into the raw PDZ. The generation of theplasma zone may be by means of a non-transfer arc plasma, rather than bymeans of a transfer arc plasma. More particularly, the high temperatureplasma zone may be provided by means of a plasma flame generated by atleast one non-transfer arc plasma gun. The zircon may be allowed to freefall through the plasma zone to achieve the dissociation into the rawPDZ, whereafter the raw PDZ may be quenched in a quench zone below theplasma zone. Preferably, three non-transfer arc plasma guns, arranged ina star fashion (when seen in plan view) with their operative ends beingdownwardly inwardly directed, may be provided, with the zircon thenbeing allowed to free fall centrally through the resultant combinedplasma zone.

The process for treating the zircon to produce the raw PDZ may thus bein accordance with WO 96/26159, which is hence incorporated herein byreference thereto.

Furthermore, the treatment of the zircon to produce the raw PDZ may formpart of the process of the present invention.

The chromophore or colour-determining agent, when it is desired toobtain a yellow pigment, may be sodium molibdate or may bepraseodymium-based, eg praseodymium oxide, carbonate or oxalate; when itis desired to obtain a blue pigment, it may be vanadium-based, eg it maybe ammonium metavanadate or vanadium pentoxide; when it is desired toobtain a pink pigment, it may be iron-based, eg it may be iron oxide oriron sulphate.

During the calcination of the base mixture, the chromophore, or atransient compound or an ion derived therefrom, becomes entrapped withinand/or around the zircon lattice, thereby forming the pigment having thedesired colour.

The mineralizers, whose function it is to reduce the temperature atwhich the reaction of the chromophore with the zircon lattice, ie thecalcination reaction, occurs, or to catalyze the calcination reaction,may be an alkali metal halide, particularly an alkali metal fluoride,any other alkaline mineralizer such as (NH₄)₂SO₄ or Na₂SO₄, or acombination of two or more of these.

The process may include forming the base mixture by mixing the raw PDZ,the chromophore and the mineralizer. The raw PDZ, chromophore andmineralizer are preferably mixed sufficiently so that the base mixtureis a homogeneous blend.

The calcination of the base mixture may be effected in an air furnace.The calcination temperature may be from 800° C. to 1300° C.

The refining of the raw pigment may include washing, comminuting anddrying it, to obtain the refined inorganic doped-zircon pigment.

The invention will now be described in more detail with reference to theaccompanying diagrammatic drawing which depicts a simplified flowdiagram of a process according to the invention for producing aninorganic doped-zircon pigment, and with reference to the subsequentnon-limiting examples.

In the drawing, reference numeral 10 generally indicates a processaccording to the invention for producing an inorganic doped-zirconpigment.

The process 10 includes a plasma reactor or plasmatron 12 which is inaccordance with WO 96/26159 (which is incorporated herein by referencethereto) and comprises three non-transfer arc plasma guns, arranged in astar pattern (when seen in plan view) with their operative ends beingdownwardly inwardly directed. In use, the guns generate a central hightemperature plasma zone which is at a temperature of at least 1800° C. Azircon feed conduit is arranged so that zircon can be allowed to freefall centrally through the plasma zone, thereby to be dissociated intoraw PDZ. A quench zone, in which the raw PDZ is quenched rapidly tobelow 500° C., is provided below the plasma zone.

A zircon feed line 14 leads into the plasma reactor 12, while a raw PDZwithdrawal line 16 leads from the reactor 12.

The line 16 leads into a mixer 20, with a mineralizer addition line 22,as well as a chromophore addition line 24, leading into the mixer. Inthe mixer, raw PDZ, mineralizers and a chromophore are mixed into ahomogeneously blended base mixture.

A base mixture withdrawal line 26 leads from the mixer 20, into acalcination furnace 30. The calcination furnace 30 is typically an airfurnace. In the furnace, the base mixture is calcined at 800° C. to1300° C., thereby to cause the chromophore to be trapped in or aroundthe crystal lattice of the zircon.

A raw pigment line 32 leads from the calcination furnace 30 to arefining stage 34 where the raw pigment is washed, comminuted and dried.

An inorganic doped-zircon pigment withdrawal line 36 leads from thestage 34.

Raw PDZ produced by dissociating zircon sand in the non-transfer arcplasmatron 12 at an average conversion rate of 90% and with a meanparticle size of 108 μm (d₅₀, as determined by a Sedigraph 5100 particlesize analyzer) was used as starting material in the examples hereunder.In the examples, two samples of this raw PDZ were, in accordance withthe invention, used directly as feed material to the calcination furnace30, without milling and/or chemical treatment, to produce Pr-yellow andV-blue pigments by using vanadium pentoxide (V₂O₅) and praseodymiumoxide (Pr₆O₁₁) respectively as chromophore.

For each of these colours, three control samples of the same 108 μm PDZwere milled down to different particle sizes by means of a wet millingprocess in a MMS Series RAPID MILL with a 300 cc porcelain milling jarusing ytria-stabilized zirconia milling media by applying the followingmethod: For Pr-yellow pigments, the three PDZ control samples weremilled down to a d₅₀ of 3.5 μm, 6.0 μm and 8.2 μm respectively (seeTable 1) as determined by a Sedigraph 5100 particle size analyzer. ForV-blue pigments, the three PDZ control samples were milled down to 3.5μm, 6.0 μm and 8.9 μm respectively (see Table 2).

The PDZ samples were mixed with the required chromophore (Pr₆O₁₁ for theyellow pigments and V₂O₅ for the blue pigments) and mineralizers asspecified in the examples, in a Y-cone tumbler mixer and thereaftercalcined at the temperature as specified, to produce pigments. Aftercalcining, the pigments were washed in boiling aqueous hydrochloric acid(HCl) to remove any excess mineralizer and chromophore, ie thechromophore, which was not incorporated into the zircon crystal lattice.The pigments of the unmilled PDZ samples (ie according to the invention)were then comminuted or deagglomerated to a d₅₀ of between 8-9 μm forthe blue pigment, and between 6-7 μm for the yellow pigment, renderingthem suitable for application to ceramic tiles. A pigment/glaze mixturewas prepared, applied to a Johnson bisque tile with a spray-gun andfired in a muffle furnace at 1080° C. with a soaking time of 5 minutes,after which colour measurements were done with a Hunterlab colourmeasuring instrument.

To benchmark the quality of the product in each example, commerciallyavailable stains were used as standards, viz ST 4032 for the yellowpigment and ST 3042 for the blue pigment. These were obtained from FerroIndustrial Products (Pty) Ltd in Vulcania, Brakpan, South Africa.

EXAMPLE 1 Praseodymium-Yellow Doped-Zircon Pigment

An amount of 1.0 mole of unmilled, untreated PDZ was mixed with 0.014moles Pr₆O₁₁, 0.2 moles NaF and 0.2 moles (NH₄)₂SO₄ in order to ensure athorough or homogeneous blend of the colour inducing or determiningmetal oxide, the mineralizers and the PDZ. The mixture was then calcinedin an air furnace at a temperature of 1050° C. and for a soaking time of2 hours after the required temperature of 1050° C. was reached, to allowthe reaction of the praseodymium oxide and the mineralizers with thedissociated zircon to take place. The resulting raw yellow pigment wasthen washed and comminuted to a d₅₀ of between 6-7 μm as measured with aSedigraph 5100 particle size analyzer. For the control samples, similaramounts of the milled, treated PDZ were blended, calcined and posttreated as for the unmilled sample, except for the comminution stepwhich was not carried out.

The results of the colour measurements according to the Hunterlabmeasurement technique for both the invention and the control pigmentsamples are given in Table 1. From these results, the advantage of usingunmilled PDZ according to the present invention as compared to milledand chemically treated PDZ before calcining can be seen clearly.

In Table 1, the b-values (positive indicates yellow on the tile) for thethree control samples vary from 37.1 (for the 3.5 μm PDZ sample), to58.8 (for the 8.2 μm PDZ sample). For the unmilled, untreated PDZ inaccordance with the invention, b=75.8. Keeping in mind that the higherthe b-value, the deeper the yellow appears on the tile, an increase of17.0 in b for the pigment using unmilled PDZ as compared to the bestcontrol sample pigment obtained from the 8.2 μm PDZ is significant.Furthermore, this value of 75.8 compares very favourable with 79.1 forthe standard yellow pigment.

The L-value, which indicates the colour depth of the tile on a scale of100 for light (white) and 0 for dark (black), of 78.6 for the unmilledPDZ prior to calcination, compares very favourably with 78.7 for thestandard, while all the controls are lighter in comparison.

TABLE 1 Example 1 PRODUCTION OF Pr-YELLOW PIGMENT FROM UNMILLED PDZAverage L a b particle size L 100-white a+: red b+: yellow Samples (μm)L 0-black a−: green b−: blue DE* Controls 3.5 82.9 −2.3 37.1 42.3 Milled6.0 81.6 −1.7 56.9 22.6 PDZ 8.2 80.4 −1.3 58.8 20.4 before calciningInvention 120 78.6 1.8 75.8 3.3 Unmilled PDZ Standard 4.5 78.7 1.3 79.1—

The deviation parameter DE*, which indicates the deviation of the colourhue and depth from the standard and which is compounded from the primarycolour parameters according to the Hunterlab protocol, is a sensitiveparameter to determine deviations from the standard and which undertypical production line conditions should ideally not exceed 1.0. DE*drops significantly from 20.4 for the best control sample, to 3.3 forthe unmilled PDZ sample prior to calcination, which clearly emphasizesthe advantage of using the unmilled, untreated PDZ in accordance withthe present invention.

EXAMPLE 2 Vanadium-Blue Doped-Zircon Pigment

The same preparation and production conditions were used as inExample 1. However, 1.0 mole of unmilled, untreated PDZ was blended with0.045 moles V₂O₅, 0.2 moles NaF, and 1.0 mole Na₂SO₄. The mixture wasthen calcined in an air furnace at a temperature of 950° C. and for asoaking time of 1 hour after the required temperature of 950° C. wasreached, to allow the reaction of the V₂O₅ and the mineralizers with thedissociated zircon to take place. In similar fashion to Example 1, theresulting raw blue pigment was washed and comminuted to a d₅₀ between8-9 μm as measured with a Sedigraph 5100 particle size analyzer. For thecontrol samples the same amount of milled, treated PDZ each wereblended, calcined and post treated as for the unmilled sample except forthe comminution step which was not carried out.

The results of the colour measurements for the V-blue pigments are givenin Table 2.

TABLE 2 Example 2 PRODUCTION OF V-BLUE PIGMENT FROM UNMILLED PDZ AverageL a B particle size L 100-white a+: red b+: yellow Samples (μm) L0-black a−: green b−: blue DE* Controls 3.5 67.8 −17.1 −6.9 22.4 Milled6.0 67.3 −16.9 −3.8 23.8 PDZ 8.9 63.2 −17.9 −3.6 20.4 before calciningInvention 120 53.8 −19.9 −13.5 6.8 Unmilled PDZ Standard 8.5 48.9 −21.8−17.7 —

The substantial advantage of using PDZ in the unmilled and untreatedcondition prior to the calcining process as compared to the milled andtreated PDZ can once again clearly be seen in the results of theHunterlab colour measurements. The more negative the b-value is, thebluer the colour of the pigment appears after application to the ceramictile. Amongst the controls b ranges from −6.9 to −3.6 as compared to−13.5 for the pigment made according to the invention, an improvement of−6.6 with respect to the best control (Table 2). The fact that the valueof −13.5 is still less than −17.7 for the V-blue standard pigment may bedue to the fact that PDZ with a conversion rate of only 90% was used.

In terms of the L-values not only was a spread of 4.6 from 63.2 to 67.8observed between the different control samples, but also the best (forthe 8.9 μm sample) is still 14.3 off the mark with respect to thestandard. In contrast, for the unmilled PDZ pigment an improvement of9.4 compared to the best control was registered while being only 4.9 offthe mark against the standard.

Observing the deviation from the standard in terms of DE* all thecontrols deviate significantly, while the deviation of 6.8 for theunmilled PDZ represents a more acceptable shift towards the colourstandard in view of the conversion rate of 90% for the PDZ used.

In conclusion, the results for Examples 1 and 2 imply that unmilledplasma dissociated zircon with a mean particle size of 103 μm produces apigment very close to the standard particularly with regard to the b-and the L-values.

EXAMPLE 3 Effect of PDZ Conversion Rate on Pigment Colour

In this example, the influence of the PDZ conversion rate on the colourof the V-blue and the Pr-yellow pigments was determined. Samples of PDZ,respectively produced at conversion rates of 95.7, 90.0, 82.4 and 74.7%,were used to produce a series of Pr-yellow (Table 3) and V-blue (Table4) pigments each. The same preparation and production conditions, andthe same quantities of raw materials, as were used in Examples 1 and 2,were used.

In Table 3 the colour measurement results for the Pr-yellow pigmentsshow that the b-value improves from 65.6 for PDZ with a conversion rateof 74.7%, to 82.3 for PDZ with a conversion rate of 95.7%. This clearlyindicates that the higher the PDZ conversion rate, the more yellow thecolour of the pigment turns out. What is more, b=82.3 for PDZ with aconversion rate of 95.7% even surpasses that of the Pr-yellow standard,a fact supported by its colour depth (L=77.3), indicating that thisyellow pigment features a deeper hue. In this context, the deviation(DE*=4.9) from the standard must be interpreted as a beneficialdeviation.

In Table 4 the influence of the PDZ conversion rate on the colour of theV-blue pigments is shown. Here b increases from −7.8 for PDZ with aconversion rate of 74.7% to −17.9 for PDZ with a conversion rate of95.7%, giving an improvement of 10.1. Again, b=−17.9 for the PDZ withhighest conversion rate equals or betters that of the V-blue standard.

Thus, the PDZ conversion rate was found to be the most importantparameter influencing the colour of the yellow and blue pigments. Higherconversion rates consistently result in deeper blue and yellow pigments,respectively, that compare very favourably with the pigment standards.Apparently a higher PDZ conversion rate implies that less unconvertedzircon is present, which reduces the susceptibility for colouring of thezircon.

TABLE 3 Example 3 COLOUR EFFECT OF THE PDZ CONVERSION RATE ON Pr-YELLOWConversion L a b rate of PDZ L = 100: white a+: red b+: yellow Samples(%) L = 0: black a−: green b−: blue DE* Standard — 78.7 1.3 79.1 —Invention 95.7 77.3 4.8 82.3 4.9 Unmilled 90.0 78.8 1.6 71.6 7.6 PDZ82.4 79.6 0.6 68.3 10.8 74.7 79.3 0.03 65.6 13.5

TABLE 4 Example 3 COLOUR EFFECT OF THE PDZ CONVERSION RATE ON V-BLUEConversion L a b rate of PDZ L = 100: white a+: red b+: yellow Samples(%) L = 0: black a−: green b−: blue DE* Standard — 49.0 −21.9 −17.3 —Invention 95.7 50.7 −20.3 −17.9 2.4 Unmilled 90.0 53.7 −19.9 −13.4 6.4PDZ 82.4 55.0 −18.9 −11.5 8.9 74.7 57.3 −18.2 −7.8 13.2

The Applicant has thus surprisingly found that it is possible to produceinorganic zircon-based pigments directly from PDZ, without firsttreating the PDZ by milling it and chemically treating it. Thesetime-consuming and costly steps can thus be eliminated.

The use of inorganic zircon-based pigments (also known as stains)particularly, but not solely, in colouring ceramic articles, eg ceramictiles, is well established.

The present invention thus provides a process whereby such pigments canbe produced more readily and more cost-effectively than has hithertobeen the case.

1. A process for producing an inorganic doped-zircon pigment, whichprocess includes calcining a base mixture comprising rawplasma-dissociated zircon, a chromophore, and at least one mineralizer,to produce a raw pigment; and refining the raw pigment to obtain aninorganic doped-zircon pigment.
 2. A process according to claim 1, whichincludes forming the base mixture by mixing the raw plasma-dissociatedzircon or PDZ, the chromophore and the mineralizer.
 3. A processaccording to claim 2, wherein the raw PDZ, chromophore and mineralizerare mixed sufficiently so that the base mixture is a homogeneous blend.4. A process according to claim 1 inclusive, wherein the chromophore orcolour-determining agent is selected from the group comprising sodiummolibdate; praseodymium oxide, carbonate or oxalate; ammoniummetavanadate; vanadium pentoxide; iron oxide and iron sulphate.
 5. Aprocess according to claim 1, wherein the mineralizer is an alkali metalfluoride, (NH₄)₂SO₄ or Na₂SO₄, or a combination of two or more of these.6. A process according to claim 1, wherein the calcination of the basemixture is effected in an air furnace.
 7. A process according to claim1, wherein the calcination temperature is from 800° C. to 1300° C.
 8. Aprocess according to claim 1, wherein the refining of the raw pigmentincludes washing, comminuting and drying it, to obtain the refinedinorganic doped-zircon pigment.